1. Field of the Invention
The present invention relates to the diversity technologies and particularly to a diversity method and apparatus for combining two received signals. The present invention also relates to the receiving techniques and particularly to a receiving method and apparatus for receiving signals via a time-varying radio channel.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been used in the wireless standards such as IEEE802.11a and HIPERLAN/2. The burst signals in such a wireless LAN are generally received via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus must carry out the channel estimation dynamically. In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided. One is the known signal, provided for all carries in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the burst signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In the field of the wireless communications, the spread-spectrum (SS) communication scheme has been considered conventionally. The SS communication scheme includes direct sequence (DS) scheme and frequency hopping (FH) scheme. In the FH scheme, the frequency of a carrier is hopped in sequence based on a code sequence so as to carry out the spread-spectrum communication. Thus, the spectrum distribution occupies a wide band when observed for a long period of time. On the other hand, the signal thereof occupies a specific frequency band only when observed for each bit or symbol and it is narrower in band than that of the DS scheme. Hence, the FH scheme is said to be an SS of interference-avoidance type. This is advantageous in that the probability under which a plurality of users communicate at the same timing is small.
The MB-OFDM scheme, in which this FH scheme and the above-mentioned OFDM modulation scheme are combined together, has been proposed and is applied to WPAN (Wireless Personal Area Network). WPAN is a wireless network whose range is narrower than the wireless LAN and is a close-range network and may be constituted by PDA and peripheral equipment, for example. In UWB (Ultra Wideband) using such a modulation scheme as MB-OFDM modulation scheme, the use of band of 3.1 GHz to 10.6 GHz is scheduled.
The first problem is as follows. The MB-OFDM modulation scheme supports a plurality of kinds of data transmission rates. Under these circumstances, if the data transmission rate is low, the same symbols are transmitted consecutively. There may be cases where the signal strength or signal-to-noise ratio for the consecutively transmitted symbols differs. In such a case, the combining gain drops. This is because if these symbols are combined as they are, the symbol whose signal strength or signal-to-noise ratio is smaller than the other symbols has an adverse effect on the synthesis.
The second problem is as follows. The radio channel through which the data is transmitted varies with time. Since the FH scheme is used in UWB, the signal strength or signal-to-noise ratio differs for each symbol. If the receiving processing based on the same reference is performed on the symbols in which the signal strength or signal-to-noise ratio differs, it is likely that the error will be caused in the symbol whose signal strength or signal-to-noise ratio is lower than the other symbols. This also makes it difficult to correct the errors even if some error-correcting method is employed, and therefore there will be cases where the quality of data transmission deteriorates.